The present invention relates to dental cold-polymerizing resin, and particularly to a dental resin composition that is much more improved in terms of color stability and curing properties. More particularly, care is taken of using a powder-liquid type of cold-polymerizing resin by brush on technic.
Even at the present time, the cold ( or normal temperature) polymerization of methyl methacrylate, etc., with organic peroxides and aromatic tertiary amines finds wide application in dentistry. To a lesser extent, the cold polymerization of methyl methacrylate, etc., with pyrimidinetrione derivatives and organometallic compounds and optionally with organic halogen compounds is applied to some products.
These dental cold-polymerizing resins are widely and diversely used as temporary crowns that are temporary substitutes for permanent crown prostheses or bridges, for repairing broken denture base or artificial teeth, and for other purposes. Generally, they are used by a technical procedure called "brush on technic". According to this "brush on technic", the tip of a small brush made up of swine or nylon bristles is first impregnated with a sufficient amount of liquid material, and then brought into contact with powder materials. The liquid incorporated in the brush enters among the powder particles forming the powder material through capillarity. As the powder comes into contact with the liquid or is mixed with the liquid, they start to react with each other. The behavior of the polymerization achieved by this procedure is considerably different from that achieved by mixing together constant amounts of the powder and liquid materials in a rubber cup with the use of a spatula or other means. This can be well understood by measuring a (approx. 0.1-0.5 g) scoop of material taken up by brush on technic for curing capability. In other words, the scoop of material is inhomogeneously cured. This phenomenon has been common to both the reaction in which organic peroxide and aromatic tertiary amine compounds participate and that in which pyrimidinetrione derivative, organometallic and organic halogen compounds participate, and has offered some difficulty, although it has been somehow averted by skillful dentists or dental technicians.
The reaction in which the pyrimidinetrione derivative, organometallic and organic halogen compounds participate yields a cured material that does not suffer from yellowing, but the reaction in which the organic peroxide and aromatic tertiary amine compounds participate yields a cured material that tends to suffer from yellowing. However, this yellowing problem is still .considered very difficult to solve technically, and so is left as it is.
Another problem with brush on technic is that air bubbles are likely to enter the cured material and, after polishing, remains in the form of impurity. Moreover, the air bubbles may act as a certain type of defects, and cause a drop in mechanical strength, which may lead to a re-fracture.
Thus, problems in connection with inhomogeneous curing caused by brush on technic, yellowing of cured material, and air bubbles brought about by brush on technic all remain unsolved.
The brush on technic of a conventional powder-liquid type of cold-polymerizing resin making use of the reaction in which organic peroxide and aromatic tertiary amine compounds participate and the reaction in which pyrimidinetrione derivative, organometallic and organic halogen compounds participate have the following problems.
1. Brush on technic makes curing inhomogeneous
Now consider a (approx. 0.1-0.5 g) scoop of material taken up by brush on technic. The capability of this scoop of material to cure can be estimated by measuring the curing time in a regulated-temperature room of 23.degree. C. by a contact procedure making use of a Vicat needle with a sectional area of 1 mm.sup.2. There is some considerable difference in the curing time between the outer and central surface portions of a scoop. Here consider a commercially available material (e.g., GC UNIFAST) containing an organic peroxide and an aromatic tertiary amine. The outer surface portion of a scoop of this material is generally cured for about 4 minutes at 23.degree. C. However, a considerable longer time that is about 10 minutes to about 20 minutes is needed to cure the central portion of a scoop.
Then, consider a scoop of an experimental material composed of a pyrimidinetrione derivative, an organometallic compound and an organic halogen compound under the same conditions. The outer surface portion of the experimental material is cured very sharply or for a short time in a matter of 3 minutes at 23.degree. C., but the central portion thereof is not cured at all.
Thus, when such inhomogeneous curing happens, it takes much time for dentists or dental technicians to make temporary crowns or repair broken denture base or artificial teeth, resulting in an increase in the time taken by dentists for dental examination and treatment and in the time needed for dental technicians works. One scoop of material starts to be cured on its outer surface, and when the material is polished at this time, the uncured central portion of the material is wound round the polishing bar, leaving white stripes. These pose an aesthetic problem in the case of temporary crowns or repaired denture base or artificial teeth; it is required to redo dental examination and treatment or dental works.
2. Yellowing of cured material
As mentioned, the cold polymerization of a radical polymerizable compound having at least one ethylenically unsaturated double bond (e.g., methyl methacrylate) by the reaction in which an organic peroxide and an aromatic tertiary amine participate yields a cured material that suffers from yellowing when the polymerization is achieved by any one of the procedures of brush on technic or mixing. This problem is known to be solved by use of a combination of three catalysts, i.e., a pyrimidinetrione derivative, an organometallic compound and an organic halogen compound, as disclosed in Japanese Patent Application No. 62-50295, entitled "Method for Curing of Dental Resin". In the case of the brush on technic, however, it is required to solve the inhomogeneous curing and yellowing problems at the same time. Never until now is there any powder-liquid type of dental cold-polymerizing resin capable of solving the inhomogeneous curing-upon-brush on technic and yellowing problems at the same time.
3. Internal air bubbles are generated when the brush on technic is used
Except when polymerized in a pressure vessel, catalysts, e.g., an organic peroxide and an aromatic tertiary amine, when used for the cold polymerization of a radical polymerizable compound having at least one ethylenically unsaturated double bond (e.g., methyl methacrylate), causes the reaction to proceed too rapidly, resulting in the generation of some considerable internal air bubbles upon brush on technic. This poses a problem when temporary crowns are polished on their surfaces or broken denture base or artificial teeth are polished on their surface after repairing, because impurity upon polishing remains in these air bubbles. The air bubbles makes the resin inferior in the mechanical strength and so causes a re-fracture, because the resin cannot produce its own properties.